Add section in FAQ for Ubuntu14.04

With the default ubuntu version (14.04) of jekyll building the site does not work. Added problem description and workaround in README.md.

Note: I see below that changes are marked for lines 45, 77. I can not figure out why that is, this PR is only for adding the section from line 316.
"it is because you are using the `jekyll` program of the ubuntu distribution (in `/usr/bin`) which is too old." -> "it is because you are using an old version of Jekyll."